A new generation of ion accelerators is being developed, such as the ion accelerator shown in U.S. Pat. No. 4,812,775, issued Mar. 14, 1989, and entitled "Electrostatic Ion Accelerator", which accelerators operate at low voltage and low power, but which operate at high currents in order to achieve critically significant saturated yields of radioisotopes required for applications such as medical imaging. The ion beams for accelerators of this type are generated in, for example, a tandem accelerator such as that shown in the aforementioned patent, which operates in a vacuum and the beams are transmitted through a window covered by a thin foil to a fluid filled target chamber, which fluid may be a gas or a liquid. The fluid filling the target chamber will depend on the isotopes being generated, and will thus vary with application.
However, low energy, high current accelerators require approximately 40 times more cooling capacity for the foil window separating the accelerator and target chambers than a high energy accelerator having comparable yields. The reasons for the more severe foil cooling requirements for low energy accelerators include (a) the fact that the lower beam energy results in the deposition of a larger fraction of the instantaneous beam power in the foil (i.e. the foil tends to absorb more of the incident energy) and (b) the higher currents required to produce clinically significant radioisotope yields for such lower energy accelerators. It has been found that for the low energy accelerators, roughly 5% to 10% of the power in the ion beam is deposited in the foil. For a 3.7 MeV, 1 mA beam, this results in about 180 watts being deposited in the foil. This is roughly the power that must be removed from the foil by cooling. As indicated above, this is approximately a factor of 40 larger than the 3-5 watts that has to be removed from the foil in a system using a 10 MeV, 50 .mu.A ion accelerator. A need, therefore, exists for a more efficient construction to remove the accumulated heat from the foil to keep the foil from disintegrating or rupturing as a result of melting or softening. chamber. Such foil window must in each instance be able to withstand the substantially pressure differential thereacross, which is normally equal to atmospheric pressure, and to dissipate the small fraction of the particle beam energy, generally 1 to 10% of such energy, that is absorbed in the foil.
A need, therefore, exists for an improved foil window construction for particle accelerators which provides significantly enhanced cooling capacity for the foil window and which results in a substantial reduction in the stresses applied to the foil.